Syndiotactic 1,2-polybutadiene is a thermoplastic elastomer having both the property of plastic (hardness) and the properties of rubber (elasticity and flexibility), and can be easily molded by use of a general-purpose polymer processing machine, while having a definite crystallinity. Accordingly, it has come to be used in various industrial goods.
In particular, it is excellent in gas permeability resistance and transparency, can be shaped without the addition of plasticizers, compared to vinyl chloride-based resins requiring a large amount of plasticizers, and has moderate flexibility and self-adhesion. Accordingly, the application thereof to medical instruments such as an infusion tube and a catheter is expanding.
However, syndiotactic 1,2-polybutadiene has a melt-starting temperature as low as 70 to 95° C., when low in crystallinity. When this is used in a medical instrument such as an infusion tube, an infusion solution container or a catheter, and sterilized by steam, a problem has been practically encountered because of its poor heat resistance in some cases.
As a method for improving the balance in performance, while keeping the characteristics of syndiotactic 1,2-polybutadiene that it is a thermoplastic elastomer having both the property of plastic (hardness) and the properties of rubber (elasticity and flexibility), there has been proposed a method of crosslinking only a surface of a molded article by irradiating it with an ultraviolet ray having a specific wavelength, thereby curing it (Japanese Patent Laid-Open Publication 2000-129017). This method exhibits a reasonable effect in respect to improvement of the balance in performance among flexibility, transparency and heat resistance (high-pressure steam sterilization resistance).
Further, in order to obtain a harder surface, electron beam irradiation methods have also been variously proposed. These methods exhibit a certain effect for scratch resistance.
However, in the molded article obtained by the method described in the above-mentioned specification, only the surface layer is crosslinked by the irradiation of the ultraviolet ray having a wavelength within the specific range. Accordingly, for example, the heat resistance of the inside of a medical tube is not necessarily fully satisfactory. Further, according to the conventional methods using electron beams, the inside of the molded article is also hardened in excess, which poses the problem that flexibility, one property of syndiotactic 1,2-polybutadiene, is almost lost.
It is therefore an object of the present invention to provide a syndiotactic 1,2-polybutadiene-containing molded article which is useful for medical applications and the like, such as an infusion tube, an infusion solution container, a catheter and the like, excellent in flexibility and hardness, and also excellent in steam sterilization resistance.